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planner: prepared plan cache support limit #12686

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Feb 16, 2023
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planner: prepared plan cache support limit #12686

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Expand Up @@ -22,7 +22,6 @@ TiDB 优化器对这两类查询的处理是一样的：`Prepare` 时将参数
- 包含 `ignore_plan_cache` 这一 Hint 的查询，例如 `select /*+ ignore_plan_cache() */ * from t`；
- 包含除 `?` 外其他变量（即系统变量或用户自定义变量）的查询，例如 `select * from t where a>? and b>@x`；
- 查询包含无法被缓存函数。目前不能被缓存的函数有：`database()`、`current_user`、`current_role`、`user`、`connection_id`、`last_insert_id`、`row_count`、`version`、`like`；
- `?` 在 `Limit` 后的查询，如 `Limit ?` 或者 `Limit 10, ?`，此时 `?` 的具体值对查询性能影响较大，故不缓存；
- `?` 直接在 `Order By` 后的查询，如 `Order By ?`，此时 `?` 表示根据 `Order By` 后第几列排序，排序列不同的查询使用同一个计划可能导致错误结果，故不缓存；如果是普通表达式，如 `Order By a+?` 则会缓存；
- `?` 紧跟在 `Group by` 后的查询，如 `Group By ?`，此时 `?` 表示根据 `Group By` 后第几列聚合，聚合列不同的查询使用同一个计划可能导致错误结果，故不缓存；如果是普通表达式，如 `Group By a+?` 则会缓存；
- `?` 出现在窗口函数 `Window Frame` 定义中的查询，如 `(partition by year order by sale rows ? preceding)`；如果 `?` 出现在窗口函数的其他位置，则会缓存；
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